1. Field of the Invention
The present invention-relates to a laser optical apparatus which is suitably applied to a laser exposure apparatus to be used in manufacturing a semiconductor integrated circuit.
2. Related Background Art
A semiconductor device such as a semiconductor integrated circuit has been increasingly microminiturized and a minimum line width of a circuit pattern is now reaching an area of quarter micron from an area of sub-micron. As a result, in a reduction projection type exposure apparatus, a projection lens having a large aperture number has been developed; but it is necessary to further shorten a wavelength of an exposure light in order to further microminiturize the semiconductor device.
To this end, an exposure apparatus which uses an excimer laser light source which generates a light of a shorter wavelength than that of an illumination light by a g-line (wavelength 435 nm) or an i-line (wavelength 365 nm) of a mercury lamp which is presently widely used in the exposure apparatus, is considered very promising.
The excimer laser light source can generate laser beams of different wavelengths by changing gas of an oscillation medium. Among others, Krypton fluoride (KrF, wavelength 248 nm) and Argon fluoride (ArF, wavelength 193 nm) which assure a stable high output and have a short wavelength are advantageous.
In order to form a finer pattern, it is desirable to use an excimer laser of as short wavelength as possible, and the excimer laser exposure apparatus which uses the ArF excimer laser having the wavelength of 193 nm rather than the KrF excimer laser having the wavelength of 248 nm is presently most promising.
However, when a laser beam of a short wavelength is passed through an optical system in an oxygen atmosphere, a cloud may appear in the optical system. The cloud of the optical system is considered to be generated by a chemical reaction of a coating on a surface of a lens in the oxygen atmosphere by the laser beam, or by the deposition to the surface of the lens of a material removed from a lens barrel by the chemical reaction.
In the KrF excimer laser exposure apparatus, it is not necessary to fill an optical path with a high transmission medium such as nitrogen gas because the absorption of a laser energy by oxygen in air when the laser beam propagates in air is negligibly small. However, when the ArF excimer laser is to be used as the exposure light source, the absorption by oxygen is significantly larger than that of KrF because the ArF excimer laser has a spectrum characteristic which substantially overlaps with an absorption band of oxygen. Thus if air (oxygen) is present in the optical path, not only the laser energy is lost but also noxious ozone gas is generated by photo-decomposition of oxygen and the transmission in the laser beam path varies with time.
In order to avoid the above problem, the laser beam path may be blocked from the atmosphere and the inside thereof may be evacuated or substituted by helium He or nitrogen N.sub.2. In this case, however, the exposure apparatus itself becomes complicated and of large scale.